Diffusion

Photo by: Lansera

Diffusion is the movement of molecules from a region of high concentration
to one of low concentration. If you have ever opened a bottle of cologne
or perfume, you have witnessed diffusion. Molecules of the scent escape
from the container, where they are present in very high concentration.
They spread outward in every direction to regions where they are in low
concentration. Your nose is able to detect the smell of the cologne or
perfume even if you are quite a distance from the bottle that has been
opened.

Diffusion occurs in all states of matter: solid, liquid, and gas. It
occurs rapidly enough to be observable in a reasonable period of time,
however, only in liquids and gases.

You can demonstrate diffusion easily in your home. Fill a glass with
water. Then add 10 drops of ink (any color) to the water very carefully.
The ink sinks to the bottom of the glass because it is more dense than
water. Place the glass in a place where it will not be disturbed and make
observations of it every day. Over time, the colored ink at the bottom of
the glass spreads upward. It moves from a region of high concentration to
one of low concentration.

Eventually, the water in the glass is the same shade: a grey, light blue,
or pink throughout. The original black, blue, or red ink has been diluted
with water to produce the paler shade. Diffusion eventually stops because
no region of high ink concentration remains. The concentration of ink and
water is the same throughout the glass. That rule applies to all cases of
diffusion. When differences in concentration no longer exist, diffusion
stops.

Osmosis

Osmosis is diffusion through a membrane. The membrane acts as a barrier
between two solutions of different concentration. One substance (usually
water) travels from an area of high concentration to one of low
concentration. Osmosis can be compared to the examples of diffusion given
above involving perfume and ink. In those cases, no barrier was present to
separate perfume from air or ink from water. Diffusion took place directly
between two materials.

In contrast, a barrier is always present with osmosis. That barrier is
usually called a semipermeable membrane because it allows some kinds of
materials to pass through, but not others.

The most familiar example of osmosis through a semipermeable membrane may
be a living cell. Cells contain semipermeable membranes

Carbon dioxide vapor diffusing from an open gas jar. The vapor
molecules are traveling from the area of high concentration (the jar)
to the area of low concentration (the open air).
(Reproduced by permission of

Photo Researchers, Inc.

)

that act something like a plastic baggy holding cell contents inside. The
cell membrane is not a solid material, however, but a thin sheet
containing many tiny holes. (Imagine a self-sealing sandwich
bag—its surface dotted with minuscule holes—then filled with
water.) The holes allow small molecules and ions (such as molecules of
water and sodium ions) to pass through, but trap larger molecules (such as
proteins) inside the cell.